Cold weather and your aircraft’s engine oil

In my February 2022 column, I discussed the differences between single grade and multigrade oils.

The big advantage for multigrade oils is cold weather starting and operation.

This brings up the question: What did they do before multigrade oils were developed?

During World War II, aircraft could be needed at a moment’s notice and the planes were parked outside in the winter weather. The solution? Dilution.

When a plane returned from a mission and was going to be down for a while, they would shut it down and when cool they would top off the oil tank and then add a measured amount of avgas to the oil. They would then run the engine for a few minutes to mix the two components.

When the plane was needed for the next mission, they would prop it over and, depending on conditions, start the engine up and go.

As the engine (or engines) warmed up, the fuel in the oil would evaporate and the oil would be back to “normal.”

I am too young to have seen this done, but when I was rewriting the mil specs during my time at Shell Oil, the diluted viscosity requirement for the straight mineral oils was deleted and this was the reason given.

Viscosity and Colder Temperatures

On the subject of the viscosity of mineral oils, ashless dispersant (AD) oils are actually multigrade oils — sort of. The dispersant in an AD oil is actually a dispersant/viscosity index improver additive.

For example, to blend Aeroshell Oil W 120, we take Aeroshell 100 mineral oil and add the AD additive. That’s all.

Why is this information important for aircraft owners?

If you are breaking in a new or rebuilt engine on mineral oil during the coldest part of the year, you should change your pre-heating criteria to account for this difference in viscosity.

For example, most engine guides recommend pre-heating at any temperature below 20°F.

If you are using a straight mineral oil grade 100, I would recommend that you pre-heat whenever the temperature is below 32°F. This will help ensure adequate oil flow once your engine starts.